Titanium sheet and method for producing the same

ABSTRACT

A titanium sheet having a sheet thickness of 0.2 mm or less and including a hardened layer on a surface, the titanium sheet including a chemical composition containing, in mass percent: Fe: 0.001 to 0.08%; and O: 0.03 to 0.08%, wherein a grain size satisfies following Formulas (1) to (3), a thickness of the hardened layer is 0.1 to 2.0 μm. The titanium sheet has both a sufficient strength and an excellent workability. Formulas (1) to (3) are dave≥2.5 (1), t/dave≥3.0 (2), and t/dmax—1.5 (3), where, in Formulas (1) to (3), t denotes the sheet thickness (μm), dave denotes an average grain size (μm), and dmax denotes a maximum grain size (μm).

TECHNICAL FIELD

The present invention relates to a titanium sheet and a method forproducing the titanium sheet. The titanium sheet refers to a titaniumsheet having, for example, a sheet thickness of 0.2 mm or less.

BACKGROUND ART

Titanium materials have high specific strengths and excellent corrosionresistances and are widely used as a wide variety of starting materialsfor industrial use such as chemical plants and building materials, andas starting materials for consumer products such as camera bodies,timepieces, and sports equipment. Sheets such as foil having thicknessesof 0.2 mm or less are used as audio components (speaker diaphragms,etc.), anti-corrosion films/sheets, and the like.

In general, there is a trend for metal materials to be required to havehigh strengths as well as to have workabilities. Titanium materials areno exception. However, in general, when titanium materials are made tohave high strengths, their workabilities drop. Thus, attempts fortitanium products have been made to optimize a balance between strengthand workability by controlling an amount of oxygen, an amount of iron, agrain size, and the like. Increase in an amount of oxygen causessolid-solution strengthening, increasing the strength. Increase in anamount of iron, which is a β phase stabilizing element, inhibits graingrowth in a phase crystal grain boundaries to thereby refine grains, andthe strength increases as well. In both cases, ductility is compromisedwith the increase in the strength, with which formability deteriorates.

With regard to a titanium foil of 25 μm thickness, JP2616181B (PatentDocument 1) describes that a good Ericksen value can be ensured byrolling a titanium foil under predetermined rolling conditions tocontrol a grain size into ASTM No. 12 to 14.

In contrast, good shape retention properties are required for titaniumfoils of 0.2 mm or less in thickness after shape working. In general,enhancing a strength of a material ensures a good shape retentionproperty. However, as described above, a problem of decreasedformability arises, and a good workability cannot be achieved.

WO 2014/027657 (Patent Document 2) discloses a titanium sheet having asheet thickness of 0.2 mm or more, wherein bulk Fe is contained at 0.1%or less by mass, O (oxygen) is contained at 0.1% or less by mass, sheetthickness / particle size 3 is established, particle size 2.5 μm issatisfied, a hardened layer is included on its surface, and a region ofthe hardened layer is at a depth of 200 nm or more and 2 μm or less fromthe surface, so that the titanium sheet is made to be excellent in shaperetention property and workability.

Documents mentioning a maximum grain size in a titanium plate includethe following documents. JP2002-012931A (Patent Document 3) describes ahot-rolled titanium plate for a surface member of an electrodepositiondrum, for which generation of coarse grains are avoided to improvegrindability but is not intended for sheets having sheet thicknesses of0.2 mm or less and makes no description about a relation between coarsegrains and workability. JP2013-095964A (Patent Document 4) ratherdescribes defining a lower limit of a number of coarse grains.JP2005-105387A (Patent Document 5) defines an upper limit of anabundance ratio of regions having grain sizes not less than 1.25 times aminimum average grain size, but the definition is for reducing macropatterns on a surface to improve a surface texture, and the documentdoes not mention improvement of workability.

LIST OF PRIOR ART DOCUMENTS Patent Document

Patent Document 1: JP2616181B

Patent Document 2: WO 2014/027657

Patent Literature 3: JP2002-012931A

Patent Literature 4: JP2013-095964A

Patent Literature 5: JP2005-105387A

SUMMARY OF INVENTION Technical Problem

The invention described in Patent Document 2 enables titanium sheetshaving sheet thicknesses of 0.2 mm or less to be produced as titaniumsheets having excellent workabilities and high strengths. Meanwhile, ithas been found that conventional titanium sheets vary in value ofelongation, which is an index of workability, degrading a balancebetween elongation and strength, with a result that a value ofelongation may be insufficient when a predetermined strength isobtained. When elongation is insufficient, a titanium sheet cannot beprocessed into an intended shape, and in order to produce processedgoods, it is necessary to bring at least an average value of elongationto a level that allows the processing. However, when there is greatvariation, the average value of elongation may fall below the level thatallows the processing. Therefore, in a case of great variation, a yieldof the processed goods drops as compared with a case of small variation.

The present invention has an objective to provide a titanium sheet thathas both a sufficient strength and an excellent workability, and amethod for producing the titanium sheet.

Solution to Problem

The gist of the present invention is as follows.

(1) A titanium sheet having a sheet thickness of 0.2 mm or less andincluding a hardened layer on a surface, the titanium sheet having achemical composition containing, in mass percent:

Fe: 0.001 to 0.08%; and

O: 0.03 to 0.08%, wherein

a grain size satisfies following Formulas (1) to (3),

a thickness of the hardened layer is 0.1 to 2.0 μm, and

a maximum height Rz is 3.0 μm or less:

d_(ave)≥2.5 (1)

t/d _(ave)≥3.0 (2)

t/d _(max)≥1.5 (3)

where, in Formulas (1) to (3), t denotes the sheet thickness (μm),d_(ave) denotes an average grain size (μm), and d_(max) denotes amaximum grain size (μm).

(2) The titanium sheet according to claim 1, wherein a ratio of grainseach having a grain size of t/2 or more is 15% or less in terms ofnumber ratio.

(3) A method for producing the titanium sheet according to the above (1)or (2) having steps of cold rolling and annealing repeated on a titaniumproduct a plurality of times, wherein the titanium product having anaverage grain size adjusted to 2.0 μm or less is finish cold rolled witha rolling ratio of 50 to 80% and thereafter finish annealed in an inertatmosphere at 570 to 750° C.

Advantageous Effects of Invention

According to the present invention, it is possible to reduce variationin elongation of a titanium sheet. As a result, a titanium sheet thathas both a sufficient strength and an excellent workability can beobtained, and the yield of processed goods can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating a relation between sheet thicknesst/average grain size d_(ave) and uniform elongation.

FIG. 2 is diagrams each illustrating metal micro-structures in a crosssection of a titanium sheet, where FIG. 2(a) illustrates a case wherecoarse grains are present, and FIG. 2(b) illustrates a case where coarsegrains are absent.

FIG. 3 is a graph illustrating a relation between sheet thicknesst/maximum grain size d_(max) and decrease of uniform elongation.

FIG. 4 is a graph of comparison of grain size distribution betweenmaterials with/without decrease in uniform elongation.

FIG. 5 is a graph illustrating a relation between coarse grain ratio anddecrease of uniform elongation.

FIG. 6 is a graph illustrating a relation between rolling ratio ofprevious cold rolling and maximum grain size d_(max)/average grain sized_(ave).

DESCRIPTION OF EMBODIMENTS

The present invention is intended for a pure-titanium sheet having asheet thickness of 0.2 mm or less. A reason for limiting the sheetthickness to 0.2 mm or less is that attaining a balance between strengthand workability for a sheet thickness of 0.2 mm or less is affected bygrain size, to which application of the present invention is highlybeneficial.

Hereafter, in the present invention, strength of a titanium sheet isevaluated in terms of 0.2% yield stress, and a workability of thetitanium sheet is evaluated in terms of uniform elongation. As to puretitanium, a value of the uniform elongation decreases as the strength(0.2% yield stress) becomes higher. Also in the present invention, atarget lower-limit value of the uniform elongation is to be changedaccording to a level of the 0.2% yield stress. Specifically, the targetlower-limit value of the uniform elongation is defined as a function ofthe 0.2% yield stress by the following Formula [4].

0.2% yield stress≤185 MPa

Uniform elongation (%)−0.4×0.2% yield stress (MPa)+85

0.2% yield stress>185 MPa

Uniform elongation (%)−0.03×0.2% yield stress (MPa)+16.5   [4]

As to pure titanium, the 0.2% yield stress increases as oxygenconcentration becomes higher or as grain size becomes smaller. The grainsize becomes larger mainly with an increase in annealing temperature oran increase in annealing duration. In contrast, the grain size becomessmaller as a content of iron increases. Therefore, to bring the 0.2%yield stress to a target value, it is important to manage an oxygenconcentration and an iron concentration in titanium, as well as tomanage conditions for intermediate annealing and finish annealing. The0.2% yield stress can be increased by controlling average grain sizewhether a grain size distribution of crystals is uniform or nonuniform.

Even a titanium sheet having a sheet thickness of 0.2 mm or lessdecreases in the elongation by refining grains, as with common findings.Meanwhile, as is clear from Patent Document 2, excessive coarseninggrains also decrease the elongation. In particular, when t/d_(ave) madeby a sheet thickness t (μm) and an average grain size d_(ave) (m) isless than 3.0, decrease in the elongation occurs. Therefore, bycoarsening grains according to a sheet thickness t of a product within arange where t/d_(ave)≥3.0, it is possible to exploit an utmostworkability of a titanium sheet having a sheet thickness of 0.2 mm orless.

In contrast, when the average grain size falls below 2.5 μm,non-recrystallized structures are likely to occur, which makes stableproduction difficult, and therefore the average grain size dave (μm) isset at 2.5 μm or more.

When pure titanium sheets having sheet thicknesses of 0.2 mm or lesswere produced with an oxygen concentration, an iron concentration, and agrain size adjusted, it has been found that there were variations invalue of elongation as described above, resulting in a poor balancebetween elongation and strength, and caused values of elongation in caseof predetermined strength were in some cases insufficient.

Hence, materials having substantially the same grain size condition werestudied in detail, and as a result of observation of cross sections oftitanium plates, there was a tendency for titanium plates having goodvalues of elongation to have no coarse grains developed, while there wasa tendency for titanium plates having poor values of elongation to havecoarse grains having large grain sizes intermixed. Next, a tension testwas conducted on JIS Class 1 pure-titanium sheets having sheetthicknesses of 0.1 mm and 0.03 mm subjected to cold rolling andannealing, to evaluate uniform elongation. FIG. 1 illustrates a graph oforganized relations between uniform elongation (%) and t/dave. Note thatdmax means maximum grain size (μm) of crystals present in a titaniumplate, and t means sheet thickness (μm). Of titanium plates havingt/d_(max) of 1.5 or more (small d_(max)), those having sheet thicknessesof 0.1 mm are marked as ◯, and those having sheet thicknesses of 0.03 mmare marked as ●, and of titanium plates having t/d_(max) less than 1.5(large d_(max)), those having sheet thicknesses of 0.1 mm are marked asΔ, and those having sheet thicknesses of 0.03 mm are marked as ▴.

First, attention will be paid to marks ◯ and ● indicating small maximumgrain sizes dmax. In FIG. 1, the marks ◯ and ● are on lines in whichuniform elongation decreases with an increase in t/d_(ave) (a decreasein d_(ave)) where t/d_(ave) of a horizontal axis is three or more. Thisis because the strength increases with decrease in the average grainsize d_(ave), and the elongation decreases accordingly. Wheret/d_(ave)<3.0, a phenomenon of decreasing elongation is seen, andtherefore t/d_(ave)≥3.0 is specified also in the present invention.

In the same FIG. 1, it is understood that marks Δ and ▴ indicating largemaximum grain sizes d_(max) show decreased elongation as compared withthe marks ◯ and ● indicating small maximum grain sizes d_(max), evenwhen compared at the same average grain size d_(ave). That is, it hasbeen found that, for titanium sheets having sheet thicknesses of 0.2 mmor less, a cause of the decrease in elongation due to variations ingrain size is presence of a coarse grain having a maximum grain sized_(max) giving t/d_(max)<1.5. FIG. 2 includes pictures each illustratingmetal micro-structures in a cross section of a titanium plate having asheet thickness of 0.03 mm, where FIG. 2(a) illustrates a case wherecoarse grains are present, and FIG. 2(b) illustrates a case where coarsegrains are absent. In addition, in FIG. 1, traces of plots of the marks◯ and ● are illustrated as solid lines. Next, for items of data on themarks Δ and ▴, differences between the items and the solid lines areevaluated at the same values of t/d_(ave), and the differences aredefined as “reduced amounts of uniform elongation (%)”. FIG. 3illustrates the marks, with its horizontal axis representing t/d_(max)and its vertical axis representing reduced amount of uniform elongation.As illustrated in FIG. 3, it has been found that decrease in elongationis not seen where t/d_(max) is 1.5 or more, whereas the uniformelongation is likely to decrease with decrease in t/d_(max) wheret/d_(max)<1.5. That is, it is found that the elongation does notdecrease if t/d_(max)≥1.5.

This is because, in a case where a number of grains in a plate-thicknessdirection is small, contribution of each grain to deformation becomessubstantial, and the elongation is influenced by deformation of onegrain. Titanium is highly anisotropic and exhibits different propertiesdepending on directions of undergoing deformation, and deformation islikely to concentrate on coarse grains having poor properties. Such arelation between sheet thickness and grain size is a phenomenon thatoccurs irrespective of sheet thickness. For example, in a case where thesheet thickness is 0.5 mm, cases giving t/d_(ave)<3.0 are those whered_(ave) is more than 160 μm, but under normal producing conditions, daveis about 100 μm or less, and such coarse metal micro-structures neverappear. However, in a case where, for example, the sheet thickness is0.2 mm, d_(ave) more than about 65 μm makes t/d_(ave)<3.0, bringingabout degradation of a property. As described above, under the normalproducing conditions, d_(ave) may become about 100 μm, and it isunderstood that t/dave has to be managed in a case where the sheetthickness is 0.2 mm or less. The same is true for t/d_(max). Therefore,the degradation of a property occurs in the sheet even in a case wherethe sheet has the same grain size distribution as that of a typicalsheet.

Hence, in the present invention, it is determined that the average grainsize d_(ave) is managed to be 2.5 or more and brought within a rangethat satisfies t/d_(ave)≥3.0 and t/d_(max)≥1.5 by inhibiting coarsegrains from being developed in a titanium plate. As a result, it ispossible to reduce variation in elongation of titanium plate and achievea good workability.

In the present invention, by specifying t/d_(max)≥1.5, it is possible toachieve good elongation as described above. In addition, grain sizedistributions were studied in detail. For titanium plates having a sheetthickness of 0.03 mm, two grain size distributions were evaluated, whichare illustrated in FIG. 4 as marks ● and ◯. Both titanium platessatisfied t/d_(max)≥1.5. As to the marks ◯, their values of uniformelongation were on the solid line of FIG. 1, not showing decrease inelongation, whereas the marks ● showed a slight decrease in elongation.A difference between both grain size distributions is that the marks ●occur with a high frequency where the grain size is 15 μm or more (t/2or more). Hence, a ratio of grains having a size of t/2 or more isdefined as coarse grain ratio (%), and a relation between the coarsegrain ratio and the “decrease of uniform elongation” is illustrated inFIG. 5, where a horizontal axis represents the coarse grain ratio and avertical axis represents the decrease of uniform elongation. Items ofdata illustrated in FIG. 5 all satisfy t/d_(max)≥1.5. As is clear fromFIG. 5, it has been found that good elongations can be achieved morestably by setting the coarse grain ratio at 15% or less. Thus, in thepresent invention, the coarse grain ratio is preferably set at 15% orless.

In the present invention, a grain size distribution is determined byobserving an L cross section under an optical microscope at a maximummagnification that allows an entire sheet thickness to be checked. Theobservation is conducted in randomly selected ten visual fields, and ineach visual field, areas of grains are calculated in a region of (sheetthickness)×(length not less than ten times sheet thickness) using imageanalysis, and diameters of the grains are calculated by approximationusing square. Using the diameters, the average grain size d_(ave) andthe maximum grain size d_(max) are calculated. In a determined grainsize distribution, the coarse grain ratio is determined as a numberratio of grains having grain sizes that are not less than sheetthickness (t)/2.

On a surface after annealing, there is carbon derived from lubricantused in the rolling, and the like, which form a hardened layer. Theformation of the hardened layer depends on amounts of elements adheredto the surface, and therefore, to remove the hardened layer totally,there is no choice but to remove a surface layer. However, the removalleads to a significant decrease in yield because of a small sheetthickness, and thus it is rather desirable to utilize this hardenedlayer. By forming the hardened layer on the surface layer within a rangethat does not cause deterioration of the workability, it is possible toprovide scratch resistance, shape retention property, or the like. Toprevent the workability from deteriorating, a thickness of the hardenedlayer needs to be 2.0 μm or less, and to provide the effect of scratchresistance or the like, the thickness needs to be 100 nm or more.

In the present invention, a maximum height Rz (JIS B 0601:2001) needs tobe 3.0 μm or less. A maximum height Rz more than 3.0 μm fails to preventfine cracks from developing on the surface, which degrades the balancebetween 0.2% yield stress and uniform elongation.

As the titanium sheet according to the present invention, use can bemade of pure titanium of JIS Class 1 or Class 2. Specifically, thetitanium sheet according to the present invention has the followingchemical composition.

For pure titanium, a required strength and an excellent ductility areachieved typically by adjusting a content of oxygen and a content ofiron. Industrially considered, oxygen is contained at 0.03% or more bymass as a lower-limit value, and 0.08% by mass is set as an upper limit.Industrially considered, iron is contained at 0.001% by mass as alower-limit value, and 0.08% by mass is set as an upper limit. With thecontent of oxygen and iron set at these ranges, by adjusting the contentof oxygen, the content of iron, and the average grain size according toa required strength level, it is possible to produce a titanium sheetthat is excellent in workability while having a required strength. Inaddition to oxygen and iron, titanium and unavoidable impurities arecontained.

As impurities, pure titanium contains nitrogen and carbon. When nitrogenand carbon fall within ranges of nitrogen: 0.001 to 0.08% by mass andcarbon: 0.001 to 0.05% by mass, respectively, which are levels ofunavoidable impurities that are normally contained, nitrogen and carbondo not have adverse effects on a quality of the titanium sheet accordingto the present invention.

Next, a method for producing a titanium sheet according to the presentinvention will be described.

In a typical process of producing a titanium sheet, a titanium productis subjected to cold rolling and annealing a plurality of times. Inparticular, annealing performed during the cold rolling is called“intermediate annealing”, cold rolling performed the last is called“finish cold rolling” and annealing performed after the finish coldrolling is called finish annealing. The intermediate annealing is a stepof subjecting a titanium product to the recrystallization after the coldrolling. Preferable conditions for each step will be described below.

Rolling ratio of finish cold rolling: 50% to 80% It is known that, as arolling ratio of the finish cold rolling increases, the average grainsize after annealing decreases and a grain size distribution can bebrought closer to uniform one. Therefore, the finish cold rolling istypically performed at least with a rolling ratio of 50% or more.However, when the finish cold rolling with a rolling ratio of 50% ormore is performed on a sheet of 0.2 mm or less in thickness, the grainsize distribution may become nonuniform. This is because, as describedabove, a number of grains in the sheet thickness direction greatlydiffers even in the same grain size distribution. When the rolling ratiois increased to obtain a more uniform grain distribution, a fine crackdevelops on a surface. In a case of a large sheet thickness, the finecrack is very small relative to the thickness, which will not degradethe property. However, in a case of a small sheet thickness, aninfluence of the fine crack cannot be ignored. Therefore, a rollingratio of a cold rolling cannot be increased. In addition, carbon derivedfrom rolling oil or the like used in the cold rolling is adhered, makingthe surface layer hard and susceptible to crack through the annealing,which makes it impossible to reduce a maximum height Rz to 3.0 μm orless, and thus it is necessary to set a rolling ratio of a finish coldrolling at 80% or less. However, only setting the rolling ratio of thefinish cold rolling at 50 to 80% is insufficient, and it is necessary toprepare for obtaining more uniform metal micro-structure before thefinish cold rolling.

Rolling ratio of cold rolling before the finish cold rolling (previouscold rolling): 30% to 80%

As described above, in a case of a titanium sheet of 0.2 mm or less inthickness, an influence of a fine crack cannot be ignored. Therefore,forming fine-grain metal micro-structures in a titanium product beforethe finish cold rolling makes it easy to place strain uniformly duringthe finish cold rolling. This is because, in titanium including coarsemetal micro-structures, strain placed by rolling is maintained throughtwin deformation, which makes it difficult to form a dislocation cell,serving as a nucleus for recrystallization. In addition, deformationoccurs on a grain basis, and therefore a nonuniform distribution ofstrain is less likely to occur when a deformation unit is small, whichmakes it easy to form uniform recrystallization nuclei.

To confirm the above, an experiment in which pure titanium of JIS Class1 was repeatedly subjected to cold rolling and annealing to be producedinto a titanium sheet was conducted. At this point, the rolling ratio ofthe finish cold rolling was set at 50%, and the rolling ratio of theprevious cold rolling was changed to various rolling ratios. Inaddition, the finish annealing and intermediate annealing before thefinish cold rolling (previous annealing) were performed in an Ar gas ata temperature of 670° C. for 10 mins. FIG. 6 illustrates a relationbetween the rolling ratio of the previous cold rolling and the maximumgrain size dmax / the average grain size d_(ave). A value of dmax/daveindicates a uniformity in strain placed during the finish cold rolling.In general, the maximum grain size is likely to be large in a portionwhere a placed strain is slight, and thus smaller d_(max)/d_(ave)indicates a strain placed more uniformly.

As illustrates in FIG. 6, the higher the rolling ratio of the previouscold rolling, the smaller d_(max)/d_(ave) becomes, and thus a strain isplaced uniformly during the finish cold rolling, allowing development ofcoarse grains to be inhibited. From this result, the rolling ratio ofthe previous cold rolling is set at 30% or more, more desirably 40% ormore, still more desirably 50% or more. Note that the rolling ratio ofthe previous cold rolling is set at 80% or less so as to prevent a crackfrom developing on a surface. This enables the maximum height Rz to beset at 3.0 μm or less.

Metal micro-structure before the finish cold rolling: Average grain sizeis set at 2.0 μm or less

As described above, controlling the rolling ratio of the cold rollingmakes it easy to obtain a uniform grain size distribution. However, onlycontrolling the rolling ratio of the cold rolling may fail to obtain astable workability. Hence, it is effective to make metalmicro-structures before the finish cold rolling fine grains,specifically, to make the metal micro-structures have an average grainsize of 2.0 μm or less because, as to strain placed by cold working,fine grains allow many dislocations to be introduced by a small amountof work. Metal micro-structures having average grain sizes of 2.0 μm orless are mixed grain structures including recrystallized grains andnon-recrystallized grains, or non-recrystallized structures. Thenon-recrystallized structures are in a phase before recrystallizationand can be considered to be smaller than recrystallization nuclei. Therecrystallization nuclei are of course smaller than recrystallizedgrains. Therefore, in a case of mixed structures includingrecrystallized grains and non-recrystallized grains, when an averagegrain size of the recrystallized grains is 2 μm or less, thenon-recrystallized structures are naturally smaller than therecrystallized grains. Also in a case where all of the structures arenon-recrystallized structures, recrystallization nuclei andrecrystallized grains grown from the non-recrystallized structures are 2μm or less in production within the scope of the present invention, andtherefore the non-recrystallized structures can be considered to havesizes smaller than the recrystallization nuclei and the recrystallizedgrains. From this assumption, the finish cold rolling is enabled, andmany dislocations (strain) can be provided even at a limited rollingratio, and by the finish annealing, it is possible to obtain grains withhigh uniformity. As a result, a stable workability can be given to thetitanium sheet.

The sheet requires, as described above, the limitation on the rollingratio and the uniformity in grain size distribution, but the uniformityto this degree is not required for normal sheets, and surface cracks tosome extent raise no problem. Therefore, the rolling ratio of the finishcold rolling can be increased to be high, and in particular, for thepurpose of reducing producing steps, the rolling ratio of the finishcold rolling is typically increased by performing recrystallizationsufficiently.

Temperature of intermediate annealing: 500 to 800° C.

As with the previous annealing, the intermediate annealing is preferablyperformed at a low temperature, at which microstructures are easilyobtained. The metal micro-structures are not necessarily refined in thisphase, but the intermediate annealing is desirably performed at 500 to700° C. to obtain microstructures stably before the finish cold rolling.However, the intermediate annealing may be performed at a temperaturehigher than such a temperature, and in this case, the intermediateannealing needs to be performed in less than one minute. It is moredesirable to perform the inteanediate annealing in less than 30 seconds,and in this manner, performing the intermediate annealing at 700 to 800°C. raises no problem.

Temperature of annealing before the finish cold rolling (previousannealing): 400 to 700° C.

A temperature of the previous annealing differs according to adifference in annealing method. In continuous annealing, it is desirableto set the temperature at 500 to 600° C. to obtain non-recrystallizedstructures. To obtain microstructures, the temperature may be 600 to700° C. However, a temperature more than these temperatures causescoarse metal micro-structures to be formed by recrystallization andgrowth, and thus the annealing is performed at 500 to 700° C. A shorterretention duration allows microstructures to be obtained, but anexcessively short retention duration makes reduction of strainaccumulated in the cold rolling insufficient, failing to obtain asufficient ductility, and thus it is preferable that the annealing isperformed for about one minute as a guideline, and the retentionduration is adjusted in consideration of a time taken for temperaturerise and a stability of the temperature. In batch annealing, atemperature distribution develops in a coil, and nonuniformity is likelyto occur, and thus a batch annealing at a low temperature for a longtime is required. Therefore, the annealing may be performed at atemperature of 400 to 550° C. for about one hour, as a guideline. Anexcessively low temperature fails to recover the ductility sufficiently,and an excessively high temperature causes coarsening and nonuniformity.

Temperature of finish annealing: 500 to 750° C.

The average grain size d_(ave) is influenced mainly by the temperatureand duration of the finish annealing, as well as a concentration of ironand a concentration of oxygen in a titanium product. Since the presentinvention specifies t/d_(ave) 3, the upper limit of d_(ave) differsaccording to the sheet thickness, and an upper limit of the temperatureof the finish annealing also differs to set t/d_(ave) 3. When the finishannealing is to be performed in an inert atmosphere at 750° C. or less,it is possible to prevent grains from being coarsened excessively.

This annealing results in different productivities according toannealing methods. The continuous annealing enables the annealing to beperformed over an entire length of a coil with stability. In addition,use can be made of carbon derived from rolling oil adhered to a surfaceto form a hardened layer, and if the hardened layer is insufficientbecause of a small amount of adhered carbon on the surface, the hardenedlayer can be formed by introducing nitrogen in an atmosphere or using amixed gas of the air and an Ar gas. However, performing the annealing inthe atmosphere or a nitrogen atmosphere makes discoloration or excessiveformation of the hardened layer likely to occur, and for stableproduction, a hardened layer formed by dispersing carbon derived fromrolling oil adhered to the surface. An annealing duration differsdepending on the temperature or a targeted grain size, and for example,annealing at 570° C. for 5 mins causes recrystallization. To furtherimprove productivity, it is desirable to perform the annealing at anannealing temperature of 600 to 750° C. In this case, performing theannealing for about 1 min can cause recrystallization.

The batch annealing is difficult to perform the annealing over an entirelength of a coil uniformly, and it is necessary to perform the annealingat a temperature as low as 500 to 570° C. for a long time, as well as toset a rate of temperature increase and a cooling rate as low aspossible, which results in a low productivity. A high temperature failsto make metal micro-structures uniform in the coil, and an excessivelylow temperature requires a still longer time for recrystallization andmay fail to cause the recrystallization. In a case of batch finishannealing, a process thereof involves a temperature rise to 500° C. for10 hours or more, retention for 10 hours or more, and thereafter coolingfor 15 hours or more. Furthermore, there is a concern that even aprocess at a low temperature for a long time may fail to control metalmicro-structures into predetermined metal micro-structures in a part ofthe coil, significantly losing yield. In addition, the annealing at alow temperature for a long time forms a thick hardened layer, theretention duration may be adjusted according to facilities, referred to15 h or less. Therefore, for a high productivity, it is desirable to usethe continuous annealing.

EXAMPLE

Pure titanium products of JIS Class 1 were repeatedly subjected to acold rolling and an annealing, to be produced into titanium sheets.Table 1 shows chemical compositions and producing conditions of thetitanium sheets.

In the table, a phrase “PREVIOUS COLD ROLLING” means cold rollingperformed before the finish cold rolling, and a phrase “INITIAL COLDROLLING” means cold rolling performed before the “previous coldrolling”. A phrase “PREVIOUS ANNEALING” means annealing performed beforethe finish cold rolling, and a phrase “INITIAL ANNEALING” meansintermediate annealing performed before the “previous annealing”.Examples of a case where annealing durations are 1 min are examplessimulating the continuous annealing, and examples of a case whereannealing durations are 1 h or more are examples simulating the batchannealing. As an annealing atmosphere, an Ar gas was used except forNos. 23 to 26 (comparative examples 10 and 11, examples 14 and 15), Nos.24 and 25 were made in a nitrogen gas, and Nos. 23 and 26 were made inthe atmosphere. Sheet thicknesses of the starting materials wereadjusted by cutting or grinding, according to sheet thicknesses afterthe finish cold rolling and the annealing.

The grain size distribution after the final annealing was determined byobserving an L cross section under an optical microscope at a maximummagnification that allowed an entire sheet thickness to be checked. Theobservation is conducted in randomly selected ten visual fields, and ineach visual field, areas of grains are calculated in a region of (sheetthickness) x (length not less than ten times sheet thickness) usingimage analysis, and diameters of the grains are calculated byapproximation using square. Using the diameters, the average grain sized_(ave) and the maximum grain size d_(max) were calculated. In adetermined grain size distribution, the coarse grain ratio wasdetermined as a number ratio of grains having grain sizes that were notless than sheet thickness (t)/2. In observation of metalmicro-structures after the previous annealing, the EBSD was used tomeasure an average grain size of recrystallized grains, with anorientation difference of 5° or more assumed to be a grain boundary. Themeasurement was performed on randomly selected five visual fields eachhaving a region of sheet thickness×length of 100 to 200 μm and separatedby 0.2 μm, with a magnification of 500× or more that allows an entiresheet thickness to be checked in a visual field.

For the uniform elongation, a tension test was conduct on an ASTM 1/2tensile test specimen taken in an L direction, at a strain rate of12%/min until rupture, and an amount of strain reaching a maximum loadpoint on an obtained nominal stress—nominal strain curve was evaluatedas the uniform elongation.

For a thickness of the hardened layer, GDS was used to perfoun ananalysis of oxygen, nitrogen, carbon, titanium, and iron in a depthdirection in a region on a surface of a sample having a diameter of 4 mmby the Ar ion-sputtering, and a thickness within which a totalconcentration of oxygen, nitrogen, and carbon is 0.5% or more by mass isdetermined as the thickness of the hardened layer. For the determinationat this point, zinc oxide (containing oxygen at 19.8% by mass) was usedfor oxygen, austenitic stainless steel (containing nitrogen at 0.3% bymass) was used for nitrogen, titanium alloy (containing carbon at 0.12%by mass) was used for carbon, and the depth was in terms of puretitanium of JIS Class 1. Results of the above are shown in Table 2.Numeric values falling out of the ranges according to the presentinvention are underlined. When a relation between the 0.2% yield stressand the uniform elongation did not meet Formula [4], an acceptancejudgement was determined to be ×.

TABLE 1 PRODUCING METHOD ROLLING CHEMICAL RATIO OF INITIAL COMPOSITIONINITIAL ANNEALING (mass %) COLD TEMPER- No. CATEGORY Fe O C N ROLLINGATURE DURATION 1 COMPARATIVE EXAMPLE 1 0.03 0.06 <0.01 <0.01 50% 620° C.1 min 2 COMPARATIVE EXAMPLE 2 0.03 0.06 <0.01 <0.01 50% 620° C. 1 min 3COMPARATIVE EXAMPLE 3 0.03 0.06 <0.01 <0.01 50% 650° C. 1 min 4COMPARATIVE EXAMPLE 4 0.03 0.06 <0.01 <0.01 50% 500° C. 1 min 5 EXAMPLEEMBODIMENT OF 0.03 0.06 <0.01 <0.01 50% 500° C. 1 min PRESENT INVENTION1 6 EXAMPLE EMBODIMENT OF 0.03 0.06 <0.01 <0.01 50% 500° C. 1 minPRESENT INVENTION 2 7 EXAMPLE EMBODIMENT OF 0.03 0.06 <0.01 <0.01 50%550° C. 1 min PRESENT INVENTION 3 8 EXAMPLE EMBODIMENT OF 0.03 0.06<0.01 <0.01 50% 550° C. 1 min PRESENT INVENTION 4 9 COMPARATIVE EXAMPLE5 0.05 0.05 <0.01 <0.01 50% 700° C. 1 min 10 COMPARATIVE EXAMPLE 6 0.050.05 <0.01 <0.01 50% 700° C. 1 min 11 COMPARATIVE EXAMPLE 7 0.05 0.05<0.01 <0.01 50% 650° C. 1 min 12 EXAMPLE EMBODIMENT OF 0.05 0.05 <0.01<0.01 50% 650° C. 1 min PRESENT INVENTION 5 13 EXAMPLE EMBODIMENT OF0.05 0.05 <0.01 <0.01 50% 650° C. 1 min PRESENT INVENTION 6 14COMPARATIVE EXAMPLE 8 0.05 0.05 <0.01 <0.01 50% 550° C. 1 min 15 EXAMPLEEMBODIMENT OF 0.05 0.05 <0.01 <0.01 50% 650° C. 1 min PRESENT INVENTION7 16 EXAMPLE EMBODIMENT OF 0.05 0.05 <0.01 <0.01 50% 650° C. 1 minPRESENT INVENTION 8 17 COMPARATIVE EXAMPLE 9 0.05 0.05 <0.01 <0.01 50%500° C. 1 min 18 EXAMPLE EMBODIMENT OF 0.05 0.05 <0.01 <0.01 50% 650° C.1 min PRESENT INVENTION 9 19 EXAMPLE EMBODIMENT OF 0.05 0.05 <0.01 <0.0150% 650° C. 1 min PRESENT INVENTION 10 20 EXAMPLE EMBODIMENT OF 0.050.05 <0.01 <0.01 50% 500° C. 1 min PRESENT INVENTION 11 21 EXAMPLEEMBODIMENT OF 0.05 0.05 <0.01 <0.01 50% 500° C. 1 min PRESENT INVENTION12 22 EXAMPLE EMBODIMENT OF 0.05 0.05 <0.01 <0.01 50% 550° C. 1 minPRESENT INVENTION 13 23 COMPARATIVE EXAMPLE 10 0.05 0.05 <0.01 <0.01 50%550° C. 1 min 24 COMPARATIVE EXAMPLE 11 0.05 0.05 <0.01 <0.01 50% 550°C. 1 min 25 EXAMPLE EMBODIMENT OF 0.05 0.05 <0.01 <0.01 50% 550° C. 1min PRESENT INVENTION 14 26 EXAMPLE EMBODIMENT OF 0.05 0.05 <0.01 <0.0150% 550° C. 1 min PRESENT INVENTION 15 27 COMPARATIVE EXAMPLE 12 0.050.05 <0.01 <0.01 50% 550° C. 1 min 28 EXAMPLE EMBODIMENT OF 0.05 0.05<0.01 <0.01 50% 550° C. 1 min PRESENT INVENTION 16 29 EXAMPLE EMBODIMENTOF 0.05 0.05 <0.01 <0.01 50% 550° C. 1 min PRESENT INVENTION 17 30EXAMPLE EMBODIMENT OF 0.05 0.05 <0.01 <0.01 50% 550° C. 1 min PRESENTINVENTION 18 31 EXAMPLE EMBODIMENT OF 0.05 0.05 <0.01 <0.01 50% 550° C.1 min PRESENT INVENTION 19 32 EXAMPLE EMBODIMENT OF 0.05 0.05 <0.01<0.01 50% 550° C. 1 min PRESENT INVENTION 20 33 COMPARATIVE EXAMPLE 130.05 0.05 <0.01 <0.01 50% 550° C. 1 min 33 COMPARATIVE EXAMPLE 14 0.030.05 <0.01 <0.01 50% 650° C. 1 min 34 COMPARATIVE EXAMPLE 15 0.03 0.05<0.01 <0.01 50% 650° C. 1 min 35 COMPARATIVE EXAMPLE 16 0.04 0.05 <0.01<0.01 50% 650° C. 1 min 36 EXAMPLE EMBODIMENT OF 0.05 0.05 <0.01 <0.0150% 650° C. 1 min PRESENT INVENTION 21 PRODUCING METHOD ROLLING METALMICRO- ROLLING RATIO OF PREVIOUS STRUCTURE RATIO OF PREVIOUS ANNEALINGIMMEDIATELY FINISH FINISH ANNEALING COLD TEMPER- BEFORE FINISH COLDTEMPER- No. ROLLING ATURE DURATION COLD ROLLING ROLLING ATURE DURATION 150% 600° C. 1 min  1.6 90% 670° C. 1 min 2 90% 600° C. 1 min  1.8 70%700° C. 1 min 3 20% 500° C. 1 min  8.0 50% 660° C. 1 min 4 15% 600° C. 1min 22.0 50% 640° C. 1 min 5 50% 520° C. 1 min NOT 50% 680° C. 1 minRECRYSTALLIZED 6 40% 520° C. 1 min NOT 50% 670° C. 1 min RECRYSTALLIZED7 50% 520° C. 1 min NOT 50% 650° C. 1 min RECRYSTALLIZED 8 50% 500° C. 1min NOT 50% 630° C. 1 min RECRYSTALLIZED 9 15% 600° C. 1 min 32.0 50%800° C. 1 min 10 50% 600° C. 1 min 22.0 50% 760° C. 1 min 11 50% 500° C.1 min NOT 50% 760° C. 1 min RECRYSTALLIZED 12 70% 500° C. 1 min NOT 50%740° C. 1 min RECRYSTALLIZED 13 70% 500° C. 1 min NOT 50% 730° C. 1 minRECRYSTALLIZED 14 70% 650° C. 1 min  6.1 50% 720° C. 1 min 15 70% 500°C. 1 min NOT 50% 700° C. 1 min RECRYSTALLIZED 16 70% 500° C. 1 min NOT50% 680° C. 1 min RECRYSTALLIZED 17 70% 650° C. 1 min  9.0 30% 720° C. 1min 18 50% 500° C. 1 min NOT 70% 750° C. 1 min RECRYSTALLIZED 19 50%500° C. 1 min NOT 70% 740° C. 1 min RECRYSTALLIZED 20 50% 600° C. 1 min 1.2 70% 730° C. 1 min 21 50% 600° C. 1 min  1.3 80% 700° C. 1 min 2250% 600° C. 1 min  1.3 80% 680° C. 1 min 23 50% 600° C. 1 min  1.4 80%650° C. 1 min 24 50% 600° C. 1 min  1.4 80% 630° C. 1 min 25 50% 600° C.1 min  1.4 80% 600° C. 1 min 26 50% 600° C. 1 min  1.4 80% 620° C. 1 min27 50% 600° C. 1 min  1.4 80% 580° C. 20 h 28 50% 600° C. 1 min  1.4 80%580° C.  4 h 29 50% 600° C. 1 min  1.4 80% 600° C.  1 h 30 50% 500° C. 2h NOT 80% 600° C.  1 h RECRYSTALLIZED 31 50% 400° C. 8 h NOT 80% 580° C. 8 h RECRYSTALLIZED 32 50% 400° C. 8 h NOT 80% 520° C. 15 hRECRYSTALLIZED 33 50% 400° C. 8 h NOT 80% 480° C. 20 h RECRYSTALLIZED 3350% 650° C. 1 min 33.0 30% 770° C. 1 min 34 50% 650° C. 1 min 33.0 30%770° C. 1 min 35 15% 600° C. 1 min 25.0 50% 770° C. 1 min 36 70% 500° C.1 min NOT 50% 700° C. 2.5 h  RECRYSTALLIZED “METAL MICRO-STRUCTUREIMMEDIATELY BEFORE FINISH COLD ROLLING” means crystal micro structureimmediately before the finish cold rolling, and its numerical valuesmean an average grain size (μm) of recrystallized grains

TABLE 2 SHEET HARDENED COARSE THICKNESS/ GRAIN SIZE/μm t/d LAYER Rz/GRAIN 0.2% PS/ UNIFORM FORMULA [4] No. CATEGORY mm AVERAGE MAXIMUMAVERAGE MAXIMUM THICKNESS/nm μm RATIO (%) MPa ELONGATION/% ACCEPTANCE 1COMPARATIVE EXAMPLE 1  0.03 8.8 13.1 3.41 2.29 120 3.2 0 211 10.1 x 2COMPARATIVE EXAMPLE 2  0.03 9.3 14.9 3.23 2.01 130 3.3 0 207 10.2 x 3COMPARATIVE EXAMPLE 3  0.03 7.1 21.3 4.23 1.41 100 1.6 15.3 228 8.9 x 4COMPARATIVE EXAMPLE 4  0.03 6.8 22.3 4.41 1.35  70 1.4 11.8 231 8.7 x 5EXAMPLE EMBODIMENT OF THE  0.03 8.6 17.4 3.49 1.72 140 1.2 6.1 213 15.6∘ PRESENT INVENTION 1 6 EXAMPLE EMBODIMENT OF THE  0.03 7.0 15.6 4.291.92 140 1.6 1.2 229 14.2 ∘ PRESENT INVENTION 2 7 EXAMPLE EMBODIMENT OFTHE  0.03 5.4 12.3 5.56 2.44 120 1.3 0 251 11.3 ∘ PRESENT INVENTION 3 8EXAMPLE EMBODIMENT OF THE  0.03 3.7 8.2 8.11 3.66 130 1.2 0 290 11.2 ∘PRESENT INVENTION 4 9 COMPARATIVE EXAMPLE 5 0.1 50.1  81.3 2.00 1.23 9301.4 36.2 127 29.6 x 10 COMPARATIVE EXAMPLE 6 0.1 30.5  74.3 3.28 1.35420 1.2 3.8 144 26.3 x 11 COMPARATIVE EXAMPLE 7 0.1 30.3  82.2 3.30 1.22500 1.2 6.5 145 24.6 x 12 EXAMPLE EMBODIMENT OF THE 0.1 26.5  59.8 3.771.67 1620  1.3 3.1 150 37.7 ∘ PRESENT INVENTION 5 13 EXAMPLE EMBODIMENTOF THE 0.1 22.3  58.3 4.48 1.72 1200  1.2 1.6 158 33.6 ∘ PRESENTINVENTION 6 14 COMPARATIVE EXAMPLE 8 0.1 17.8  67.3 5.62 1.49 630 1.3 0162 20.1 x 15 EXAMPLE EMBODIMENT OF THE 0.1 13.1  35.1 7.63 2.85 320 1.30 185 28.3 ∘ PRESENT INVENTION 7 16 EXAMPLE EMBODIMENT OF THE 0.1 10.7 30.5 9.35 3.28 300 1.4 0 198 29.6 ∘ PRESENT INVENTION 8 17 COMPARATIVEEXAMPLE 9 0.1 25.9  76.7 3.86 1.30 730 1.6 15.2 151 20.6 x 18 EXAMPLEEMBODIMENT OF THE 0.2 32.1  61.2 6.23 3.27 840 1.7 0 143 39.8 ∘ PRESENTINVENTION 9 19 EXAMPLE EMBODIMENT OF THE 0.2 23.5  52.2 8.51 3.83 7001.8 0 155 32.3 ∘ PRESENT INVENTION 10 20 EXAMPLE EMBODIMENT OF THE 0.221.4  52.8 9.35 3.79 680 1.1 0 160 32.2 ∘ PRESENT INVENTION 11 21EXAMPLE EMBODIMENT OF THE 0.2 12.8  30.6 15.63  6.54 500 1.5 0 187 27.1∘ PRESENT INVENTION 12 22 EXAMPLE EMBODINENT OF THE 0.2 10.2  28.119.61  7.12 300 1.2 0 201 24.7 ∘ PRESENT INVENTION 13 23 COMPARATIVEEXAMPLE 10 0.2 10.6  32.6 18.87  6.13 2520  1.3 0 193 10.5 x 24COMPARATIVE EXAMPLE 11 0.2 8.8 21.6 22.73  9.26 2150  1.4 0 199 9.9 x 25EXAMPLE EMBODIMENT OF THE 0.2 5.6 18.3 35.71  10.93  1720  1.1 0 24014.2 ∘ PRESENT INVENTION 14 26 EXAMPLE EMBODIMENT OF THE 0.2 8.4 72.223.81  2.77 1930  1.1 0 215 21.9 ∘ PRESENT INVENTION 15 27 COMPARATIVEEXAMPLE 12 0.2 24.6  56.8 8.13 3.52 2310  1.4 0 148 24.6 x 28 EXAMPLEEMBODIMENT OF THE 0.2 16.1  31.3 12.42  6.39 1820  1.4 0 159 28.4 ∘PRESENT INVENTION 16 29 EXAMPLE EMBODIMENT OF THE 0.2 20.3  44.6 9.854.48 1520  1.5 0 151 28.9 ∘ PRESENT INVENTION 17 30 EXAMPLE EMBODIMENTOF THE 0.2 20.2  39.6 9.90 5.05 1390  1.2 0 153 29.8 ∘ PRESENT INVENTION18 31 EXAMPLE EMBODIMENT OF THE 0.2 18.4  28.1 10.87  7.12 1420  1.1 0161 29.4 ∘ PRESENT INVENTION 19 32 EXAMPLE EMBODIMENT OF THE 0.2 4.6 8.143.48  24.69  1060  1.7 0 261 9.8 ∘ PRESENT INVENTION 20 33 COMPARATIVEEXAMPLE 13 0.2 2.4 4.9 83.33  40.82  1040  1.2 0 340 5.9 x 33COMPARATIVE EXAMPLE 14 0.2 44.3  138.6 4.51 1.44 760 1.2 13.9 130 31.6 x34 COMPARATIVE EXAMPLE 15 0.4 45.2  141.3 8.85 2.83 790 1.4 0 131 43.1 ∘35 COMPARATIVE EXAMPLE 16 0.5 41.4  113.1 12.08  4.42 690 1.3 0 130 42.2∘ 36 EXAMPLE EMBODIMENT OF THE 0.2 62.1  131.4 3.22 1.52 1920  1.3 16.2145 38.1 ∘ PRESENT INVENTION 21

For both of example embodiments of the present invention and comparativeexamples, their finish annealing temperatures were changed within therange according to the present invention and the average grain sizes arechanged to obtain various strengths as the 0.2% yield stress. As afinish annealing temperature was increased, an average grain size becamelarger, a 0.2% yield stress decreased, and a value of uniform elongationincreased.

As to all of example embodiments 1 to 4 of the present invention (sheetthickness 0.03 mm), example embodiments 5 to 8 of the present invention(sheet thickness 0.1 mm), and the present inventions 9 to 20 (sheetthickness 0.2 mm), their chemical compositions and producing conditionsfell within the respective ranges specified in the present invention,and d_(ave) 2.5 μm, t/d_(ave)≥3, t/d_(max)≥1.5, and thickness ofhardened layer: 0.1 to 2.0 μm were satisfied. As a result, both of their0.2% yield stresses and uniform elongations satisfied Formula [4], andthus good uniform elongations according to strength levels weresuccessfully obtained.

As to a comparative example 1, its rolling ratio of the finish coldrolling was as high as 90%, and while its index for the grain sizedistribution was satisfied, its maximum height Rz was more than 3.0 andFormula [4] was not satisfied due to a fine crack on its surface. As toa comparative example 2, its rolling ratio of a last intermediaterolling was 90%, and its maximum height Rz was more than 3.0 μm, andFormula [4] was not satisfied due to a fine crack on its surface.

As to comparative examples 3 to 6, and 8, since their rolling ratios ofthe last intermediate rolling were small, or their grains were coarsenedby a last intermediate annealing, their metal micro-structures beforethe finish cold rolling were coarse, and their coarse grain ratios didnot satisfy t/d_(max)≥1.5, failing to satisfy Formula [4]. In addition,the results of comparative examples 5 and 6 were also due to their highfinish annealing temperatures, which easily coarsened grains.

As to a comparative example 7, its finish annealing temperature washigh, which easily coarsened grains, failing to satisfy Formula [4]. Asto a comparative example 9, its metal micro-structures were coarsebefore the finish cold rolling, and its rolling ratio of the finish coldrolling was low, which makes uniformity of grains insufficient, failingto satisfy Formula [4].

As to a comparative example 10, an example embodiment 15 of the presentinvention, a comparative example 11, and an example embodiment 14 of thepresent invention, their hardened layers are intentionally formed byperforming the annealing in the atmosphere in the comparative example 10and the example embodiment 15 of the present invention and in thenitrogen atmosphere in the comparative example 11 and the exampleembodiment 14 of the present invention. As to a comparative example 13,the annealing was performed in vacuum for a long time to disperse carbonderived from rolling oil remaining on its surface, forming the hardenedlayer. As to comparative examples 10 to 12, their hardened layers hadthicknesses of 2μm or more, and their elongations were poorer than thoseof example embodiments 14 and 15 of the present invention, failing tosatisfy Formula [4].

As to a comparative example 12, its finish annealing duration was long,with the result that its hardened layer was formed thick. The annealingat this temperature needs to be performed in a shorter time. As to acomparative example 13, its finish annealing temperature was low, andits grain size was less than 2.5 μm even when the annealing wasperformed for 20 hours.

As to comparative examples 14 and 15, their production was under thesame conditions, but their sheet thicknesses were different from eachother. As to the comparative example 14, its sheet thickness was 0.2 mm,and its producing method did not meet the ranges according to thepresent invention, failing to satisfy Formula [4]. However, as to thecomparative example 15, its sheet thickness was 0.4 mm, and its propertydid not deteriorate even when its producing method did not meet theranges according to the present invention. As to a comparative example16, similarly, its sheet thickness is large, and its property did notdeteriorate even when its producing method is out of the rangesaccording to the present invention.

1. A titanium sheet having a sheet thickness of 0.2 mm or less andincluding a hardened layer on a surface, the titanium sheet having achemical composition containing, in mass percent: Fe: 0.001 to 0.08%;and O: 0.03 to 0.08%, wherein a grain size satisfies following Formulas(1) to (3), a thickness of the hardened layer is 0.1 to 2.0 μm, and amaximum height Rz is 3.0 μm or less: d_(ave)≥2.5 (1) t/d_(ave)≥3.0 (2)t/d_(max)≥1.5 (3) where, in Formulas (1) to (3), t denotes the sheetthickness (μm), d_(ave) denotes an average grain size (μm), and dmaxdenotes a maximum grain size (μm).
 2. The titanium sheet according toclaim 1, wherein a number ratio of grains each having a grain size oft/2 or more is 15% or less.
 3. A method for producing the titanium sheetaccording to claim 1 having steps of cold rolling and annealing repeatedon a titanium product a plurality of times, wherein the titanium producthaving an average grain size adjusted to 2.0 μm or less is finish coldrolled with a rolling ratio of 50 to 80% and thereafter finish annealedin an inert atmosphere at 570 to 750° C.
 4. A method for producing thetitanium sheet according to claim 2 having steps of cold rolling andannealing repeated on a titanium product a plurality of times, whereinthe titanium product having an average grain size adjusted to 2.0 μm orless is finish cold rolled with a rolling ratio of 50 to 80% andthereafter finish annealed in an inert atmosphere at 570 to 750° C.